The present invention relates to a new and improved method of producing titanium dioxide.
In its more particular aspects, the present invention relates to a new and improved method for producing titanium dioxide and during which method titanium slag is used as the starting material and mixed with sulphuric acid having a concentration of at least 95% by weight. Warm, concentrated sulphuric acid of a concentration less than 95% by weight is obtained from a recycling operation during the titanium dioxide production and is subsequently added to the mixture of titanium slag and the highly concentrated sulphuric acid. As a result of such treatment or extraction operation, there is obtained a solution which is suitable for the hydrolysis of the titanyl sulphate which is contained therein, after separation of the solution from the titanium slag treatment residue. This solution is subjected to hydrolysis and the hydrolysis product which is further processed for producing the desired titanium dioxide, is separated from the mother liquor. The mother liquor constitutes a weak sulphuric acid containing metal sulphates. This mother liquor is concentrated to a sulphuric acid concentration in the range of 76% by weight to 87% by weight and recycled into the titanium slag treatment. The metal salts or sulphates are separated from the mother liquor during the concentrating operation.
A method such as known, for example, from "Ullmanns Encyklopadie der technischen Chemie", 4th Edition (1979), Volume 18, pages 570 to 580, utilizes ilmenite containing relatively high amounts of iron or titanium slag as the starting material for producing titanium dioxide. The titanium slag is particularly advantageous and obtained from ilmenite by separation of the iron contained therein.
According to a further known method using titanium slag as the starting material, such as the method described, for example, in European Patent Publication No. 0,197,507, published Oct. 15, 1986, the ecologically harmful dumping or open sea disposal of the large quantities of low concentration waste sulphuric acid and metal salts are intended to be avoided. Such low concentration sulphuric acid and metal salts are obtained as by-products during the titanium dioxide production using the sulphuric acid treatment of titanium slag. The dumping or open sea disposal of these by-products are avoided by reprocessing the low concentration waste sulphuric acid and recycling the same to the greatest possible extent for re-use in the sulphuric acid treatment of the titanium slag.
The titanium slag treatment requires high starting concentrations of sulphuric acid of approximately 90% by weight and temperatures of approximately 120.degree. C. Since the mother liquor from the hydrolysis has a sulphuric acid concentration of approximately 23% by weight, this weak sulphuric acid must be concentrated to a very high extent and any disturbing metallic compounds or metal sulphates have to be separated during such concentrating operation. Appropriate methods are listed in "Materialien 2/76, Rukstande aus der Titandioxid-Produktion" (Roughly translated as: Materials 2/76, Residues from Titanium Dioxide Production), issued by the German Federal Environment Office (Umweltbundesamt), Berlin 1982, and are also known, for example, from German Patent Publication No. 2,807,380, published Mar. 29, 1979, German Patent Publication No. 2,807,394, published Mar. 29, 1979, and German Patent Publication No. 2,807,360, published Apr. 19, 1979.
When utilizing such methods, the concentration of the sulphuric acid obtained during the concentrating operation, must increase with increasing amounts of recovered sulphuric acid which are intended to be recycled. In such case, the starting concentration of sulphuric acid must be obtained by adding the required amount of still higher concentrated fresh sulphuric acid having a concentration above 95% by weight. However, particularly high final concentrations in the concentrating operation can only be realized at high energy consumption which is undesirable and also renders the method uneconomical. Consequently, the known processes do not yet operate at an optimum extent of sulphuric acid recycling in a simultaneously economical and energy-saving manner, i.e. either only a limited amount of the weak sulphuric acid, which is obtained as a result of the hydrolysis, can be recycled or the recycled sulphuric acid must be concentrated in an uneconomical manner.
In a further method such as known, for example, from European Patent Publication No. 0,031,507, published July 8, 1981, a mixture of titanium slag and ilmenite is treated or extracted using sulphuric acid having a concentration in the range of 86% by weight to 96% by weight. In order to enable re-use of a large quantity of the weak sulphuric acid, which is obtained as a result of the hydrolysis such weak sulphuric acid must be concentrated to very high concentrations in an energy-intensive manner and, preferably, fuming sulphuric acid from an external source has to be added.
In a still further method as described, for example, in European Patent Publication No. 0,220,583, published May 6, 1987, the weak sulphuric acid obtained from the method of producing titanium dioxide, is concentrated to sulphuric acid having a concentration in the range of 70% by weight to 85% by weight. Such recovered sulphuric acid is mixed with sulphuric acid having a concentration above 95% by weight, preferably fuming sulphuric acid from an external source, for re-use in the treatment or extraction of the starting material. This method also does not recycle the weak sulphuric acid in an optimum manner and with optimum energy consumption.